Secondary batteries such as lithium ion secondary batteries have characteristics such as compact size, light weight, high energy-density, and the ability to be repeatedly charged and discharged, and are used in a wide range of applications. Therefore, studies have been carried out in recent years with the objective of further raising the performance of secondary batteries through improvement of electrodes and other battery components.
An electrode for a secondary battery, such as a lithium ion secondary battery, normally includes a current collector and an electrode mixed material layer formed on the current collector. The electrode mixed material layer is formed, for example, by applying a slurry composition onto the current collector and then drying the applied slurry composition. The slurry composition contains a dispersion medium and, dispersed therein, an electrode active material, a binder composition containing a binder, and so forth.
In recent years, in order to further improve the performance of secondary batteries, much activity has been focused toward improving binder components in binder compositions used in electrode mixed material layer formation.
In one example, PTL 1 reports that excellent binding capacity is achieved by a copolymer latex that is obtained through emulsion polymerization of a monomer composition containing an aliphatic conjugated diene monomer, an alkyl (meth)acrylate monomer having an alkyl group with a carbon number of 1-3, an ethylenically unsaturated carboxylic acid monomer, and an aromatic vinyl monomer and/or a vinyl cyanide monomer in prescribed proportions, and that has a number average particle diameter and a degree of swelling in diethyl carbonate that are within prescribed ranges. PTL 1 proposes a technique for improving secondary battery high-rate discharge characteristics and charge/discharge cycle characteristics by forming an electrode using a binder composition containing the above-described copolymer latex.
In another example, PTL 2 proposes a technique for forming an electrode mixed material layer having increased homogeneity and excellent binding capacity with a current collector by using, as a binder for an electrode, a copolymer latex that is obtained through emulsion polymerization of a monomer composition containing an aliphatic conjugated diene monomer, an ethylenically unsaturated carboxylic acid monomer, and other monomers copolymerizable therewith in prescribed proportions and for which a filtration residue remaining on a 400-mesh sieve is no greater than 0.01 weight % relative to 100 weight % of solid content of the copolymer latex.